Today, steel is made by melting and refining iron and steel scrap in a metallurgical furnace. Typically, the furnace is an electric arc furnace (EAF) or basic oxygen furnace (BOF). With respect to the EAF furnaces, the furnace is considered by those skilled in the art of steel production to be the single most critical apparatus in a steel mill or foundry. Consequently, it is of vital importance that each EAF remain operational for as long as possible.
Structural damage caused during the charging process affects the operation of an EAF. Since scrap has a lower effective density than molten steel, the EAF must have sufficient volume to accommodate the scrap and still produce the desired amount of steel. As the scrap melts it forms a hot metal bath in the hearth or smelting area in the lower portion of the furnace. As the volume of steel in the furnace is reduced, however, the free volume in the EAF increases. The portion of the furnace above the hearth or smelting area must be protected against the high internal temperatures of the furnace. The vessel wall, cover or roof, duct work and off-gas chamber are particularly at risk from massive thermal, chemical, and mechanical stresses caused by charging and melting the scrap and refining the resulting steel. Such stresses greatly limit the operational life of the furnace.
Historically, the EAF was generally designed and fabricated as a welded steel structure which was protected against the high temperatures of the furnace by a refractory lining. In the late 1970's and early 1980's, the steel industry began to combat operational stresses by replacing expensive refractory brick with water-cooled roof panels and water-cooled sidewall panels located in portions of the furnace vessel above the smelting area. Water-cooled components have also been used to line furnace duct work in the off-gas systems. Existing water-cooled components are made with various grades and types of plates and pipes. An example of a cooling system is disclosed in U.S. Pat. No. 4,207,060 which uses a series of cooling coils. Generally, the coils are formed from adjacent pipe sections with a curved end cap which forms a path for a liquid coolant flowing through the coils. This coolant is forced through the pipes under pressure to maximize heat transfer. Current art uses carbon steel and stainless steel to form the plates and pipes.
In addition, today's modern EAF furnaces require pollution control to capture the off-gases that are created during the process of making steel. Fumes from the furnace are generally captured in two ways. Both of these processes are employed during the operation of the furnace. One form of capturing the off-gases is through a furnace canopy. The canopy is similar to an oven hood. It is part of the building and catches gases during charging and tapping. The canopy also catches fugitive emissions that may occur during the melting process. Typically, the canopy is connected to a bag house through a non-water cooled duct. The bag house is comprised of filter bags and several fans that push or pull air and off-gases through the filter bags to cleanse the air and gas of any pollutants.
The second manner of capturing the off-gas emissions is through the primary furnace line. During the melting cycle of the furnace, a damper closes the duct to the canopy and opens a duct in the primary line. This is a direct connection to the furnace and is the main method of capturing the emissions of the furnace. The primary line is also used to control the pressure of the furnace. This line is made up of water cooled duct work as temperatures can reach 4000° F. and then drop to ambient in a few seconds. The gas streams generally include various chemical elements including hydrochloric and sulfuric acids. There are also many solids and sand type particles. The velocity of the gas stream can be upwards of 150 ft./sec. These gases will be directed to the main bag house for cleansing as hereinabove described.
The above-described environments place a high level of strain on the water cooled components of the primary ducts of the EAF furnace. The variable temperature ranges cause expansion and contraction issues in the components which lead to material failure. Moreover, the dust particles continuously erode the surface of the pipe in a manner similar to sand blasting. Acids flowing through the system also increase the attack on the material, additionally decreasing the overall lifespan.
Concerning BOF systems, improvements in BOF refractories and steelmaking methods have extended operational life. However, the operational life is limited by, and related to, the durability of the off-gas system components, particularly the duct work of the off-gas system. With respect to this system, when failure occurs, the system must be shut down for repair to prevent the release of gas and fumes into the atmosphere. Current failure rates cause an average furnace shut down of 14 days. As with EAF type furnaces, components have historically been comprised of water-cooled carbon steel or stainless steel type panels.
Using water-cooled components in either EAF or BOF type furnaces has reduced refractory costs and has also enabled steelmakers to operate each furnace for a greater number of heats then was possible without such components. Furthermore, water-cooled equipment has enabled the furnaces to operate at increased levels of power. Consequently, production has increased and furnace availability has become increasingly important. Notwithstanding the benefits of water-cooled components, these components have consistent problems with wear, corrosion, erosion and other damage. Another problem associated with furnaces is that as available scrap to the furnace has been reduced in quality, more acidic gases are created. This is generally the result of a higher concentration of plastics in the scrap. These acidic gases must be evacuated from the furnace to a gas cleaning system so that they may be released into the atmosphere. These gases are directed to the off-gas chamber, or gas cleaning system, by a plurality of fume ducts containing water cooled pipes. However, over time, the water cooled components and the fume ducts give way to acid attack, metal fatigue or erosion. Certain materials (i.e., carbon steel and stainless steel) have been utilized in an attempt to resolve the issue of the acid attack. More water and higher water temperatures have been used with carbon steel in an attempt to reduce water concentration in the scrap and reduce the risk of acidic dust sticking to the side walls of a furnace. The use of such carbon steel in this manner has proven to be ineffective.
Stainless steel has also been tried in various grades. While stainless steel is less prone to acidic attack, it does not possess the heat transfer characteristics of carbon steel. The result obtained was an elevated off-gas temperature and built up mechanical stresses that caused certain parts to fracture and break apart.
Critical breakdowns of one or more of the components commonly occurs in existing systems due to the problems set forth above. When such a breakdown occurs, the furnace must be taken out of production for unscheduled maintenance to repair the damaged water-cooled components. Since molten steel is not being produced by the steel mill during downtime, opportunity losses of as much as five thousand dollars per minute for the production of certain types of steel can occur. In addition to decreased production, unscheduled interruptions significantly increase operating and maintenance expenses.
In addition to the water cooled components, corrosion and erosion is becoming a serious problem with the fume ducts and off gas systems of both EAF and BOF systems. Damage to these areas of the furnace results in loss of productivity and additional maintenance costs for mill operators. Further, water leaks increase the humidity in the off-gases and reduce the efficiency of the bag house as the bags become wet and clogged. The accelerated erosion of these areas used to discharge furnace off-gases is due to elevated temperatures and gas velocities caused by increased energy in the furnace. The higher gas velocities are due to greater efforts to evacuate all of the fumes for compliance with air emissions regulations. The corrosion of the fume ducts is due to acid formulation/attack on the inside of the duct caused by the meetings of various materials in the furnaces. The prior art currently teaches of the use of fume duct equipment and other components made of carbon steel or stainless steel. For the same reasons as stated above, these materials have proven to provide unsatisfactory and inefficient results.
A need, therefore, exists for an improved water-cooled furnace panel system and method for making steel. Specifically, a need exists for an improved method and system wherein water cooled components and fume ducts remain operable longer than existing comparable components.